Firing ranges are typically used by persons practicing munitions firing, including military, law enforcement, sportsmen, and recreational users. When such a firing range has been used heavily or for an extended period, using ordinary metallic bullets or projectiles, the area near the firing range can become dangerous because of the presence of large numbers of expended rounds embedded in the ground in the target area. These expended rounds can create danger by providing a hard surface from which new rounds can ricochet in an unpredictable and dangerous manner. The expended rounds can be removed by, for example, bulldozing although at a large expense, particularly when the practice range is extensive, as in the case of a military aerial practice range.
The danger from ricochets are not limited to ricochets caused by expended rounds. Projectiles such as bullets can ricochet from the ground or from a target even when the firing range is substantially free of expended rounds. For this reason, it is often desirable that practice munitions disintegrate upon striking the ground or upon striking a target.
A further difficulty with extensive use of ordinary metallic projectiles on a firing range occurs when a target is provided for practice purposes. In military or law enforcement practice, the targets often comprise expendable or dummy objects such as vehicles, tanks, buildings, etc. Extensive use of such a target eventually results in destruction of the target, requiring replacement.
Many attempts have been made to provide a projectile which is frangible, i.e. which fractures or disintegrates upon striking a target or the ground or, in some cases, upon exiting the gun muzzle. Attempts at producing a frangible or practice projectile have included projectiles composed of or including compacted metal powder (U.S. Pat. No. 3,463,047, issued Aug. 26, 1969 to Germerschausen; U.S. Pat. No. 3,338,167, issued Aug. 29, 1967 to Karlsruhe; and U.S. Pat. No. 3,123,003, issued Mar. 3, 1964 to De Jarnett, et al.), plastics or plastic composites (U.S. Pat. No. 4,108,074, issued Aug. 22, 1978 to Billing, Jr., et al.; U.S. Pat. No. 3,902,683, issued Sept. 2, 1975 to Bilsbury; U.S. Pat. No. 4,040,359, issued Aug. 9, 1977 to Blajda, et al.), epoxies or resins (U.S. Pat. No. 4,508,036, issued Apr. 2, 1985 to Jensen, et al.), and cement (U.S. Pat. No. 4,109,579, issued Aug. 29, 1978 to Carter). U.S. Pat. No. 2,926,612, issued Mar. 1, 1960 to Olin, discloses an aluminum projectile with an aluminum oxide coating about 10 microns in thickness.
None of these materials have been found satisfactory for economically producing a projectile having he ballistic characteristics necessary for realistic practice. A non-metallic projectile which closely mimics the ballistics of an ordinary metallic projectile possesses a number of characteristics. The nonmetallic projectile must have a total mass and a center of mass similar to the replaced metallic projectile. The surface characteristics of the non-metallic projectile must be similar to that of a metallic projectile so that the aerodynamics of the metallic projectile are mimicked. The non-metallic projectile must be sufficiently strong and tough to withstand thermal stress and mechanical stress such as the acceleration and torque forces created during firing and trajectory. The non-metallic projectile must also have sufficient wear and corrosion resistance that it is not erroded by frictive contact with dust or sand particles, rain drops, and the like and is not ablated or vaporized at the temperatures created by air friction during normal trajectory. A projectile which is eroded, ablated or vaporized will undergo a change in mass, center of mass, and/or surface characteristics and its ballistic characteristics will therefore be altered.
In addition to the dangers caused by ricochets, conventional metallic projectiles present a number of other difficulties, whether the projectiles are to be used for target or practice uses or are to be used as ordinary munitions. Metallic munitions can contribute to environmental contamination or deterioration. Metallic projectiles such as steel, or particularly lead projectiles, can affect the environment by, e.g., leaching into the ground water or by wild life ingestion such as ingestion of shot by waterfowl.
A further problem of metallic projectiles in general is their susceptibility to corrosion. Projectiles are often stored for a substantial period of time and exposed to the ambient atmosphere which can have high levels of humidity and acidic or otherwise corrosive components. Further, munitions are often transported through particularly corrosive environments such as salt spray or fog environments, extremely hot or cold environments, and so forth. Ordinary metallic projectiles may require coating or other steps to minimize corrosion, often with only partial success.
Ceramics are among materials which are known to, in general, have good corrosion resistance. Ceramics have not, however, found use as munitions projectiles because of the difficulty of producing a ceramic which is sufficiently inexpensive that it can be used in place of traditional metallic projectiles and which is able to survive the stresses experienced during storage, transport, and loading as well as during firing and trajectory. During transport, for example, cartridges, shells, and other munitions are often subjected to rough handling of a type which causes many conventional types of ceramics to develop cracks or other flaws. These cracks or flaws may not be visibly detectible but may cause the ceramic to fail during firing or trajectory. A munitions projectile is subjected to a number of environments or phases during its firing and trajectory, each phase having different stress characteristics. Specifically, the projectile stress environment is different for the projectile firing, travel through the barrel, trajectory through the air, and impact phases. The magnitude and type of stress during each phase depends on a number of characteristics including gun characteristics (e.g. caliber, rifling, length of barrel, etc.), type of propellant (e.g. slow burn, fast burn, etc.), projectile shape (e.g. ogive shape, bourrelet shape, driving band shape, etc.), trajectory medium (low altitude versus high altitude atmosphere, water, vacuum), and target (ground, solid target, etc.).
In the firing environment, the projectile initially experiences thermal and mechanical shock loading. Detonation sends a compressive shock wave through the projectile which, when reflected, applies tensile stresses to the projectile. Rotation of the projectile also loads the projectile in tension Thermal stresses due to temperature gradients also load the projectile in tension, shear and compression. When the tensile and compressive stresses exceed the respective strengths of the projectile, cracks develop and/or grow in the projectile. When these cracks propagate to a critical size, the projectile fails. It has been found that one of the most important stress considerations is the tensile stress at muzzle velocity. Muzzle velocity depends on a number of factors including caliber, propellant type, gun type and others. For example, a 28 centimeter (11 inch) shell may have a muzzle velocity of about 3000 feet per second (about 900 meters per second). A 20 millimeter projectile may have a muzzle velocity of about 2700 feet per second (about 800 meters per second). Muzzle velocities of 4000 ft/sec (1200 m/sec) are rarely exceeded, although velocities of up to about 5300 ft/sec (1600 m/sec) can be attained using special projectile configurations such as a small projectile fitted in a larger propelling base. Lower muzzle velocities are often encountered in connection with low caliber guns. Typical shotgun projectiles may, e.g., have a muzzle velocity of about 1200 ft/sec (360 m/sec) or lower. In general, higher muzzle velocities require higher chamber pressure and result in higher projectile stress. As an example of chamber pressure, the projectile from a 50 caliber artillery shell may be propelled with a maximum chamber pressure of 2800 kg/cm.sup.2 or more. As an example of magnitude of stress, a 20 millimeter projectile weighing 200 grams which reaches a velocity of 2700 feet per second, 5 milliseconds after detonation, undergoes a tensile stress of approximately 210 Megapascals (MPa). The tensile stress undergone by such a projectile upon striking a solid target can be on the order of 840 MPa or more.
Selection of a material, particularly a ceramic material suitable as a munitions projectile, however, cannot be accomplished merely by consideration of the stresses discussed above. Rather, the selection of a suitable material is complicated by a number of factors.
First, the intended use of the projectile must be considered. For example, different materials would be suitable for a projectile which must disintegrate upon exiting the muzzle as opposed to a projectile suitable for target or practice use which should survive until impact. Moreover, disintegration of ceramic materials under stress is best understood as a probabilistic phenomenon, i.e. for a given ceramic projectile material, designed to withstand a particular stress value, a certain number of projectiles of that material will disintegrate under a lower stress load, while a certain percentage will survive under significantly higher stress loads. When the desired use is, for example, target firing, the projectile material must be of such a nature that the percentage of projectiles which survive firing and trajectory stresses is high enough that there is not an unacceptable level of wasted materials or time yet the ceramic material must not have so great a strength that an unacceptable percentage of projectiles survives target impact. The level of performance which is acceptable depends, of course, on the intended application. In applications where safety of the user can be critical, such as in military or law enforcement applications, a lower failure rate would be considered acceptable as compared to applications such as hunting, sports competition, or other recreational applications. In general, failure rate should not exceed about 100 parts per million. For more critical uses such as military uses, failure rate should be less than about 50 parts per million, preferably less than about 10 parts per million and most preferably less than 5 parts per million. By failure of the projectile is meant that the projectile disintegrates prematurely, for example, upon firing or travel through a barrel or during trajectory, before striking a target.
In evaluating failure rates, consideration should be given not only to stresses created during firing, trajectory and impact, but also deterioration of projectiles which might occur previous to firing and thus have an impact on firing and post-firing failure. Specifically, projectiles can be subjected to deterioration during storage and transport, and particularly the jarring and shocks associated with handling the projectiles, corrosion and other deterioration which accompanies exposure to humidity, corrosive environments, heat and cold, and stresses which might occur during loading of the projectile into the gun. Although the pre-firing stresses may not produce visible or detectable changes in the projectile, they may result in unobserved microscopic flaws which contribute to projectile structural failure upon or after firing. Failure rate is most realistically evaluated by considering the effect of such pre-firing stresses.
Second, traditional or conventional ceramic materials are very often characterized by an inverse relationship between susceptibility to thermal stress and susceptibility to mechanical stress. Moreover, conventional ceramics such as alumina, mullite, cordierite, porcelain, and so forth normally have insufficient strength and toughness to survive firing and flight environments particularly in relation to high velocity guns.
Third, in order to provide a projectile which mimics the aerodynamic and trajectory characteristics of the corresponding metallic projectile, as well as mimicking the handling, feeding, and loading characteristics of the corresponding metallic projectile, it is desired to use a material which has a density similar to the density of metallic projectiles preferably on the order of about 5 grams/cc or more.
Fourth, because the response of a material to stress, abrasion and the like can be characterized by a large number of properties or measurements, including properties such as hardness, flexural strength at a variety of temperatures, coefficients of thermal expansion and conductivity, shear, bulk, and Young's moduli, Poisson's ratio, stress intensity factor, tensile strength, compressive strength, Weibull modulus, and so forth, it is no straightforward matter to select a material which will provide the characteristics desired for a projectile considering the above three factors. This is particularly true since many of the values for physical parameters are not known or readily available for the conditions to which a bullet will be subjected, such as high loading rates and accelerations, high temperatures and high pressures.
Fifth, because a bullet and a cartridge containing a bullet are subjected to a large range of temperatures, the ceramic should not have thermal expansion characteristics which are so different from those of the material from which the cartridge or other components, e.g. a driving band, are made (typically metals) that the fit between the ceramic and other components becomes either too tight or too loose in response to changes in temperature.